Source : Webster's Revised Unabridged Dictionary (1913)

Chiltern Hundreds \Chiltern Hundreds\ [AS. Chiltern the
   Chiltern, high hills in Buckinghamshire, perh. Fr. ceald cold
   + ern, [ae]rn, place.]
   A tract of crown land in Buckinghamshire and Oxfordshire,
   England, to which is attached the nominal office of steward.
   As members of Parliament cannot resign, when they wish to go
   out they accept this stewardship, which legally vacates their
   seats.